The present invention relates to load-pulled electronic measurement and characterization systems.
Extensive work by the present inventors has shown that load-pulled oscillators have important new capabilities for measurement and characterization. See U.S. Pat. Nos. 4,862,060, 4,996,490, and 5,025,222, and PCT applications WO 91/00997 (published Jan. 24, 1991) and WO 91/08469 (published Jun. 13, 1991); all of which are hereby incorporated by reference. This "load-pull" technology provides an economical measurement technique which as improved sensitivities by 100.times. to 1000.times. over any prior instrumentation for measurement of microwave phase. This capability makes possible microwave measurements which were proposed in the 1950s but will now have not been available due to the lack of accurate technology. The ground work which was extensively formulated during the early years of microwave spectroscopy may now be utilized to bring about generations of products using this simple but powerful technique.
The previous work has established many of the basic principles, but has focused primarily on monitoring continuous streams of fluids (liquids or gas phase). The inventions disclosed herein provide new concepts for "sampling" the electrical characteristics of a wide variety of materials.
Background: The "Load-Pull" Effect
It is well known to electrical engineers generally (and particularly to microwave engineers) that the frequency of an RF oscillator can be "pulled" (i.e. shifted from the frequency of oscillation which would be seen if the oscillator were coupled to an ideal impedance-matched pure resistance), if the oscillator sees an impedance which is different from the ideal matched impedance. This, a varying load impedance may cause the oscillator frequency to shift..sup.1 FNT .sup.1 Any electrical oscillator can be "pulled" to some extent--that is, its frequency can be shifted--by changing the net impedance seen by the oscillator. However, in many systems which use oscillators, pulling of a resonant circuit's frequency is undesirable. An oscillator which is too easily pulled may be overly susceptible to irrelevant external circumstances, such as changes in parasitic capacitance due to human proximity or temperature change. Normal techniques to avoid oscillator pulling include using isolation/buffering circuits between the oscillator and the variable load, and/or using a high-Q tuned circuit to stabilize the oscillator.
The present invention sets forth various innovative methods and systems which take advantage of this effect. In one class of embodiments, an unbuffered.sup.2 RF oscillator is loaded by an electromagnetic propagation structure which is electromagnetically coupled, by proximity, to a material for which real-time monitoring is desired. The net complex impedance.sup.3 seen by the oscillator will vary as the characteristics of the material in the electromagnetic propagation structure varies. As this complex impedance changes, the oscillator frequency will vary. Thus, the frequency variation (which can easily be measured) can reflect changes in density (due to bonding changes, addition of additional molecular chains, etc.), ionic content, dielectric constant, or microwave loss characteristics of the medium under study. These changes will "pull" the resonant frequency of the oscillator system. Changes in the medium's magnetic permeability will also tend to cause a frequency change, since the propagation of the RF energy is an electromagnetic process which is coupled to both electric fields and magnetic fields within the transmission line. FNT .sup.2 An unbuffered oscillator is a oscillator without buffer amplifiers or attenuators. Amplifiers boost the output power and provide isolation from the load impedance changes. Attenuators decrease the amplitude while providing an isolation of two times the attenuation. In the load pulled oscillator configuration the oscillator feedback path that supplies the phase shift needed for oscillation is separated from the load. FNT .sup.3 A "complex" number is one which can be written as A+Bi, where A is the number's "real" part, B is the number's "imaginary" part, and i.sup.2 =-1. These numbers are added according to the rule FNT (A+Bi)+(C+Di)=(A+C)+(B+D)i, FNT and are multiplied according to the rule FNT (A+Bi)(C+Di)=(AC-BD)+(AD+BC)i. FNT Complex numbers are used in representing many electrical parameters. For example, impedance can be represented as a complex number whose real part is the resistance, and whose imaginary part is equal to the reactance (inductance or capacitance). FNT Similarly, permittivity can be represented as a complex number whose imaginary part represents resistive loss, and whose real part represents reactive loading, by the medium, of the propagating electromagnetic wave.
Background: Properties of a Dielectric in a Transmission Line
To help explain the use of the load-pull effect in the disclosed innovations, the electromagnetics of a dielectric-loaded transmission line will first be reviewed. If a transmission line is (electrically) loaded with a dielectric material, changes in the composition of the dielectric material may cause electrical changes in the properties of the line. In particular, the impedance of the line, and the phase velocity of wave propagation in the line, may change.
This can be most readily illustrated by first considering propagation of a plane wave in free space. The propagation of a time-harmonic plane wave (of frequency f) in a uniform material will satisfy the reduced wave equation EQU (.gradient..sup.2 +k.sup.2)E=(.gradient..sup.2 +k.sup.2)H=0,
where
E is the electric field (vector), PA1 H is the magnetic field (vector), and PA1 .gradient..sup.2 represents the sum of second partial derivatives along the three spatial axes. PA1 k is a wave propagation vector which points in the direction of propagation and has a magnitude equal to the wave number k, and PA1 .omega.=Angular Frequency=2.pi.f. PA1 k.sub.0 =.omega./c PA1 .mu..sub.0 =Magnetic Permeability of vacuum (4.pi..times.10.sup.-7 Henrys per meter), PA1 .epsilon..sub.0 =Electric Permittivity of vacuum ((1/36.pi.).times.10.sup.-9 Farads per meter), and PA1 c=Speed of light=(.mu..sub.0 .epsilon..sub.0).sup.-1/2 =2.998.times.10.sup.8 meters/second. PA1 k=.omega./(c(.mu..sub.r .epsilon..sub.r).sup.1/2) PA1 .mu..sub.r =Relative Permeability of the material (normalized to the permeability .mu..sub.0 of a vacuum), and PA1 .epsilon..sub.r =Relative Permittivity of the material (normalized to the permittivity .epsilon..sub.0 of a vacuum). PA1 1. Refractometry PA1 2. Photoelectric Calorimetry PA1 3. Some spectrophotometry PA1 4. Some polarimetry PA1 5. Melting/softening points PA1 6. Viscometry PA1 7. Conductivity PA1 8. Some Chromatography PA1 9. Titrations PA1 10. Mass/Loss gravimetric methods PA1 11. Solvent extractions techniques
This equation can be solved to define the electric field vector E, at any point r and time t, as EQU E(r,t)=E.sub.0 exp[i(k.multidot.r-.omega.t)],
where
In a vacuum, the wave number k has a value "k.sub.0 " which is
=.omega.(.mu..sub.0 .epsilon..sub.0).sup.1/2, PA2 =.omega.(.mu..sub.0 .mu..sub.r .epsilon..sub.0 .epsilon..sub.r).sup.1/2,
where
However, in a dielectric material, the wave number k is not equal to k.sub.0 ; instead
where
This, if the relative permeability .mu..sub.r and/or the relative permittivity .epsilon..sub.r vary, the wave number k and the wave propagation vector k will also vary, and this variation will typically affect the load pulled oscillator frequency..sup.4 FNT .sup.4 The full analysis of wave propagation in a cavity or at a boundary is much more complex, but in any case wave propagation will depend on the wave number, and the foregoing equations show how the wave number k can vary as the medium changes. See generally, e.g., R. Elliot, Electromagnetics (1996); J. Jackson, Classical Electrodynamics (2d ed. 1975); G. Tyras, Radiation and Propagation of Electromagnetic Waves (1969); R. Mittra & S. Lee, Analytical Techniques in the Theory of Guided Waves (1971); L. Lewin, Theory of Waveguides (1975); all of which are hereby incorporated by reference.